A digital copying machine as an example of digital image forming apparatus forms an image through the following processes. First, an image on a document is read by a scanner, and the resulting image data is temporarily stored in an image memory. Then, the image data is read from the image memory, thereby visualizing the image data through the copying process.
However, the described conventional digital copying machine has not succeeded in giving a sufficient solution to suppress the adverse effects from the trouble occurred during operation, such as paper being stuck, errors in reading from and writing in the image memory, etc. Also, the solution to efficiently restart the operation has not been given.
For example, in the digital copying machine which employs a volatile memory composed of a semiconductor, if some trouble has occurred such as paper being stuck, etc., during the operation, and the main power of the copying machine is turned OFF, the contents stored in the picture memory would be erased. Therefore, after the trouble is settled, in order to restart the copying operation by switching the power ON, it is required to restore the image data by reading the image on the document by the scanner, thereby presenting the problem that a long time is required before the copying machine can be restarted. The described problem becomes obvious especially in the processes of inputting image data read by an external device to be temporarily stored in the image memory and of reading the image data so as to produce copies.
On the other hand, Japanese Laid-Open Patent Publication No. 146566/1988 (Tokukaisho 53-146566) discloses a digital copying machine provided with a non-volatile memory as an external memory. In this copying machine, the content in the memory, i.e., the image data, can be stored even when the power switch is turned OFF. However, the solution to efficiently restart the copying machine has not yet been given.
The described conventional digital copying machines also present the following problems. When producing copies by reading image data from the image memory, if an error has occurred during the reading operation from the picture memory, a normal memory access may not be ensured, or the reading operation may be repeated until the reading operation is permitted from the area the trouble has occurred. Therefore, the problem is presented in that the copying operation must be terminated, or an extremely long time is required for a copying operation. Namely, when a reading error has occurred in the conventional digital copying machine, an adverse effect from the trouble cannot be prevented, and thus the copying operation cannot be continued. The above-mentioned problems may occur also when a writing error has occurred during wiring operation of the image data in the image memory by reading from the document using a scanner.